The forward shift of wheel loads under braking application causes rear wheels to lockup prematurely, especially during emergency braking, leading to significantly reduced stopping distance as well as possible vehicle instability. From a braking performance point of view, the wheel slip ratio should be maintained around the peak or stable braking force range, as in the case of ABS operation. More generally however, during braking there is a significant amount of load shift between the wheels, resulting in considerably reduced handling and braking performance as well as vehicle instability. Ideally, the impacts of wheel load variations should be eliminated or compensated so as to maintain consistent vehicle dynamic response and performance.
In the area of braking control leading to enhanced braking performance and vehicle directional stability, braking control systems usually are feedback-based systems that respond to sensed wheel slip conditions to adjust individual wheel cylinder pressure. FIG. 1 illustrates a conventional antilock braking system (ABS) 10 including a master cylinder 12, brake caliper 14, wheel speed sensor 16, ABS controller 18, and pressure modulator or valve 20. Generally, the master cylinder 12 supplies pressurized fluid to the pressure modulator or valve 20 that, under normal conditions, passes the pressurized fluid to the calipers 14 that close upon a brake disc 22 to slow the associated wheel. The wheel speed sensor 16 supplies wheel speed signals to the ABS controller 18, which monitors the speed signals for a rapid deceleration indicative of wheel lockup. When such a rapid deceleration is sensed, the ABS controller 18 activates the pressure modulator 20 to a second position in which pressure to the calipers 14 is reduced, until a wheel acceleration is sensed by the ABS controller 18 at which point the pressure modulator 20 is moved toward the first position. The ABS controller 18 may cycle the pressure modulator 20 between the first and second positions (or positions intermediate the first and second positions) several times per second.
In antilock brake systems, loads applied to individual wheels are recognized only by their effect upon wheel slip conditions. Thus, the control of the individual line pressure is not directly linked to the corresponding wheel load variation, instead only the wheel slip variation. The controller may, as a result, have limited effectiveness in terms of quickness and accuracy in compensation and achieving smooth pressure application in the wheel cylinder.
An improved method and apparatus for vehicle braking more directly taking into account the impacts of wheel load is desired.